add AllKinds to the public API. Use it as a single source of truth.

kinds.go

@@ -1,6 +1,10 @@
 package lottery
 
-import "fmt"
+import (
+	"fmt"
+	"slices"
+	"strings"
+)
 
 type Kind string
 
@@ -12,22 +16,43 @@ const (
 	KindPowerball    Kind = "powerball"
 )
 
-func ParseKind(kind string) (Kind, error) {
-	switch kind {
-	case "lotto":
-		return KindLotto, nil
-	case "euromillions":
-		return KindEuroMillions, nil
-	case "setforlife":
-		return KindSetForLife, nil
-	case "thunderball":
-		return KindThunderball, nil
-	case "powerball":
-		return KindPowerball, nil
-	default:
-		return "", fmt.Errorf(
-			"invalid lottery kind: %s, must be one of: lotto, euromillions, setforlife, thunderball, powerball",
-			kind,
-		)
-	}
+var allKinds = []Kind{
+	KindLotto,
+	KindEuroMillions,
+	KindSetForLife,
+	KindThunderball,
+	KindPowerball,
+}
+
+var allKindsText = []string{
+	string(KindLotto),
+	string(KindEuroMillions),
+	string(KindSetForLife),
+	string(KindThunderball),
+	string(KindPowerball),
+}
+
+var allKindsCSV = strings.Join(allKindsText, ", ")
+
+// AllKinds returns the complete, ordered list of supported lottery kinds.
+//
+// The returned slice is a copy and can be modified safely by callers.
+func AllKinds() []Kind {
+	kinds := make([]Kind, len(allKinds))
+	copy(kinds, allKinds)
+	return kinds
+}
+
+// ParseKind parses a string into a Kind, returning an error if the string is not a valid kind.
+func ParseKind(kind string) (Kind, error) {
+	parsed := Kind(kind)
+	if slices.Contains(allKinds, parsed) {
+		return parsed, nil
+	}
+
+	return "", fmt.Errorf(
+		"invalid lottery kind: %s, must be one of: %s",
+		kind,
+		allKindsCSV,
+	)
 }

kinds_test.go

@@ -0,0 +1,71 @@
+package lottery
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestAllKindsReturnsExpectedOrder(t *testing.T) {
+	t.Parallel()
+
+	got := AllKinds()
+	want := []Kind{
+		KindLotto,
+		KindEuroMillions,
+		KindSetForLife,
+		KindThunderball,
+		KindPowerball,
+	}
+
+	if len(got) != len(want) {
+		t.Fatalf("expected %d kinds, got %d", len(want), len(got))
+	}
+
+	for i := range want {
+		if got[i] != want[i] {
+			t.Fatalf("kind at index %d: expected %q, got %q", i, want[i], got[i])
+		}
+	}
+}
+
+func TestAllKindsReturnsCopy(t *testing.T) {
+	t.Parallel()
+
+	first := AllKinds()
+	first[0] = "mutated"
+
+	again := AllKinds()
+	if again[0] != KindLotto {
+		t.Fatalf("expected AllKinds to return a defensive copy")
+	}
+}
+
+func TestParseKindAcceptsAllKinds(t *testing.T) {
+	t.Parallel()
+
+	for _, kind := range AllKinds() {
+		parsed, err := ParseKind(string(kind))
+		if err != nil {
+			t.Fatalf("expected no error for %q, got %v", kind, err)
+		}
+		if parsed != kind {
+			t.Fatalf("expected %q, got %q", kind, parsed)
+		}
+	}
+}
+
+func TestParseKindIncludesAllKindsInError(t *testing.T) {
+	t.Parallel()
+
+	_, err := ParseKind("invalid")
+	if err == nil {
+		t.Fatalf("expected an error for invalid kind")
+	}
+
+	message := err.Error()
+	for _, kind := range AllKinds() {
+		if !strings.Contains(message, string(kind)) {
+			t.Fatalf("expected error message to contain kind %q: %s", kind, message)
+		}
+	}
+}